<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div id="app">
      姓:<input type="text" v-model="firstName" /><br />
      名:<input type="text" v-model="lastName" /><br />
      全名: {{fullName}}
      <input type="text" v-model="fullName" /><br />
    </div>
    <script src="vue.js"></script>
    <script>
      new Vue({
        el: '#app',
        data() {
          return {
            firstName: 'helen',
            lastName: 'yao',
            fullName: '',
          }
        },
        watch: {
          firstName(value) {
            console.log('firstName变化' + value)
            this.fullName = value + '' + this.lastName
          },
          lastName(value) {
            console.log('lastName变化' + value)
            this.fullName = this.firstName + '' + value
          },
        },
      })
    </script>
  </body>
</html>
